The objectives of the Multicenter Study of Hydroxyurea in sickle cell disease will be to determine whether there are any long-term adverse effects of treatment of sickle cell anemia with hydroxyurea. Untoward effects of hydroxyurea could be reflected in mortality, adverse reproductive outcomes, the development of additional serious medical complications, impairment or failure of specific organs or systems (i.e. kidney, liver, central nervous system).